Hybrid battery power source for implantable medical use

ABSTRACT

A hybrid battery power source for implantable medical use provides a generally constant low internal resistance during discharge and avoids voltage delays of the type that develop as a result of run down-induced resistance increase in Li/SVO cells. The hybrid battery power source utilizes two batteries or cells, one being a primary cell of relatively high energy density and the other being a secondary cell of relatively low internal resistance that is rechargeable. The primary and secondary cells are connected in a parallel arrangement via a voltage boost/charge control circuit that is powered by the primary cell and adapted to charge the secondary cell while limiting charge/discharge excursions thereof in a manner that optimizes its output for high energy medical device use. The energy storage capacitors of the medical device in which the hybrid battery power source is situated are driven by the secondary cell. The primary cell is used to as an energy source for recharging the secondary cell.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/350,921, filed on Jan. 24, 2003 now U.S. Pat. No. 6,909,915and entitled “Hybrid Battery Power Source For Implantable Medical Use.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to improvements in the performance ofimplantable defibrillators, implantable cardioverter-defibrillators(ICDs) and other battery powered medical devices designed to providehigh energy electrical stimulation of body tissue for therapeuticpurposes.

2. Description of Prior Art

High energy battery powered medical devices, such as implantabledefibrillators and ICDs, are designed to produce a strong electricalshock to the heart when called upon to correct the onset oftachyarrhythmia. The shock is produced by one or more energy storagecapacitors that have been charged to a high voltage by the device'sbattery power source. The power source is typically a lithium/silvervanadium oxide (Li/SVO) battery or cell of the type disclosed in U.S.Pat. No. 5,458,997 of Crespi, and references cited therein. Crespi notesthat the Li/SVO chemistry is useful for defibrillation applicationsbecause of its ability to produce pulses of energy that can charge thehigh voltage capacitors within the short time frame required by thedevice. In particular, the Li/SVO battery is typically called upon tocharge the capacitors to deliver within 10 seconds or less a shock of upto 40 Joules. This must be done several times in succession ifadditional shocks are required. Unfortunately, as noted by Crespi, aLi/SVO cell can experience unpredictable resistance increase uponlong-term discharge service. In particular, Li/SVO cells commonly have atwo-stage run down with slightly different voltage plateaus at eachstage. It is at the interval between the two plateaus where it is commonto see the resistance increase described by Crespi. The problem isfurther explained in U.S. Pat. No. 6,426,628 of Palm et al. as being atransient phenomenon that occurs following a period of low current draw.When a load is reapplied (e.g., a defibrillation pulse is required), theresistance build-up temporarily prevents the cell from developing itsfull open circuit voltage potential. This condition, which is referredto as “voltage delay,” continues for a brief period until the resistancediminishes back to some nominal level.

In many cases, the voltage delay experienced by a Li/SVO cell issignificant enough to impair the cell's ability to charge the capacitorsof a defibrillator or ICD in a timely manner. This may result,prematurely, in a decision being made that the Li/SVO cell has reachedend of service (EOS) and needs to be explanted for replacement. Inaddition to the patient inconvenience and risk entailed by theexplantation procedure, a significant portion of the capacity of theLi/SVO cell is needlessly rendered unavailable for long-term use. Evenif it is not removed, the cell's operation is unpredictable, thus makingany attempt to calculate the EOS point rather complicated.

Additional shortcomings in the application of Li/SVO cells have beenpreviously identified in U.S. Pat. No. 5,674,248 of Kroll et al. Atypical Li/SVO cell suitable for the described applications has anenergy density of about 0.4 watt-hours per kilogram (Wh/kg) as comparedto 0.8 Wh/kg for lithium/carbon monofluoride (Li/CFx) cells and 0.9Wh/kg for lithium/iodide (Li/I) cells, the latter being used almostexclusively for implantable pacemakers. This energy density disadvantagerequires the use of a battery with greater volume and weight than wouldotherwise be needed if the Li/CFx or Li/I cells could be used. However,the Li/CFx and Li/I cells are unsuitable for the described applicationsbecause they cannot support the rapid discharge rates required forcharging the defibrillator capacitors. The Li/SVO cell has the addeddisadvantage of significantly higher cost when compared to the Li/CFxand Li/I chemistries.

The Kroll et al. patent propose a staged energy concentration system forproviding improved energy sources and device performance. A first energystage utilizes a Li/CFx or Li/I battery to implement a primary powersource. The first energy stage provides power to a second energy stagethat utilizes either a lithium-based rechargeable secondary battery or ahigh energy density capacitor system. Energy is transferred at alow-rate from the first energy stage comprising the primary battery tothe second energy stage comprising the rechargeable battery or the highenergy density capacitor system. A trickle charge control circuit and avoltage doubler circuit are alternatively shown being interposed betweenthe first and second energy stages. The second energy stage is rapidlydischarged upon the detection of fibrillation to develop the highvoltage charge needed for defibrillation therapy.

A shortcoming of the Kroll et al. high energy density capacitor systemis the volume and number of capacitors needed for the second stage tosupport the storage of energy required, typically 200–300 Joules for aseries of five therapeutic countershocks which might be required in aspan of less than one minute. A shortcoming of the rechargeable batterysystem is that the lithium-based battery chemistries proposed for thesecondary energy stage are not all suitable for the proposedapplication. Table 1 below sets forth the proposed chemistries. One isan LiMnO₂ system, but this is a primary system and is not suited torecharging. Another is an LiSO₂ system, but this operates with a sealedcell at a pressure of 3 to 6 atmospheres and is not suited for high-ratedischarge applications. The remaining identified chemistries, namelyLiMoS₂, LiV₂O₅, LiTiS₂, LiV₆O₁₃, LiCuC₁₂, NiCad, Alkaline and Lead acid,have not found wide acceptance in the implantable device market.

TABLE 1 Second Stage Candidate Cells Identified in U.S. Pat. No.5,674,248 Chemistry Cell Voltage (VDC) LiMoS₂ 1.85 LiMnO₂ 3.0 LiV₂O₅ 2.8LiTiS₂ 2.2 LiV₆O₁₃ 2.3 LiCuC₁₂ 3.2 LiSO₂ 3.1

A broader shortcoming is the failure of the Kroll et al. patent toidentify a specific selection for a first stage battery and aconfiguration for the identified trickle charge control circuit or thevoltage doubler circuit. A Li/I battery with a beginning of life opencircuit voltage of 2.8 volts DC is identified within the disclosure as apotential candidate for the first energy stage. The output voltage ofthis cell falls to about 2.6 volts at EOS so the cell would be incapableof charging LiMnO₂, LiV₂O₅, LiCuC₁₂ and LiSO₂ cells unless the tricklecharge control circuit utilized a means of increasing the first stageoutput voltage. This is clear because each of these cells has a higheroperating voltage than the Li/I battery. The voltage doubler circuit isused in conjunction a first stage primary battery continuouslyrecharging a second stage rechargeable battery. The Kroll et al.disclosure does not identify a method or means of controlling the flowof energy while recharging that is necessary to prevent damage to orcatastrophic failure of the second stage cell or cells.

A need therefore exists for improvement in defibrillator/ICD batterypower systems so as to overcome the above-described deficiencies of theprior art.

SUMMARY OF THE INVENTION

The foregoing problems are solved and an advance in the art is providedby a novel hybrid battery power source for high energy battery poweredmedical devices, such as implantable defibrillators and ICDs. The hybridbattery power source has relatively constant charge time characteristicsand is not affected by the voltage delay phenomenon associated withLi/SVO batteries. In addition, the hybrid battery power source providesa significant improvement in the stored energy density for animplantable power source that is suitable for high energy batterypowered medical devices. Finally, the hybrid battery power sourceovercomes the limitations of staged energy conversion systems that havebeen previously disclosed.

In exemplary embodiments of the invention, the power source utilizes twobatteries, each of which may comprise one or more cells. The firstbattery is a primary (nonrechargeable) battery of relatively high energydensity. The second battery is a secondary (rechargeable) battery whoseinternal resistance is relatively low and stable over time. The primarybattery and the secondary battery are connected in a parallelarrangement via a charge control circuit. The charge control circuit mayinclude a voltage boost function and is adapted to limit thecharge/discharge excursions of the secondary battery in a manner thatoptimizes its output for high energy medical device use.

It is therefore an object of the present invention to minimize theeffect of the change in internal resistance as a battery for implantablemedical use discharges during service.

A further object of the invention is to add a secondary power segmenthaving low internal resistance and relatively constant terminal voltageto an implantable power source that will provide a source for rapidcharging of energy storage capacitors used to deliver high energyimpulses.

A still further object of the invention is to optimize the performanceof a secondary power segment for use in a high energy medical deviceenvironment.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages of the invention will beapparent from the following more particular description of preferredembodiments of the invention, as illustrated in the accompanyingDrawings in which:

FIG. 1 is a diagrammatic perspective view showing primary and secondarybatteries in accordance with the invention;

FIG. 2 is a schematic diagram showing an exemplary hybrid battery powersource comprising a primary segment battery “A” and a secondary segmentbattery “B” connected by a voltage boost/charge control circuit havingan inductor and a pulse generating control circuit therein.

FIG. 3A is a graph which shows the relationship between energy capacityand the number of charge/discharge cycles for a typical lithium-ioncell.

FIG. 3B is a graph which shows the relationship between cell internalresistance and the number of charge/discharge cycles for a typicallithium-ion cell.

FIG. 4 is a schematic diagram showing another exemplary hybrid batterypower source comprising a primary segment battery “A” and a secondarysegment battery “B” connected by another voltage boost/charge controlcircuit having a flyback transformer and a pulse generating controlcircuit therein.

FIG. 5 is a schematic diagram showing a hybrid battery power source ofthe invention arranged in a defibrillator and discharging into a voltageamplification system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Introduction

Exemplary hybrid battery power sources for use with implantabledefibrillators, ICDs and other high energy battery powered medicaldevices will now be described, together with an exemplary defibrillatorthat incorporates a hybrid battery power source therein. As indicated byway of summary above, the power source embodiments disclosed herein arecharacterized by having primary and secondary batteries. The primarybattery has high energy density but typically will also have internalresistance that is relatively high, or which can increase significantlyduring discharge. Examples of this primary battery include thelithium-carbon monofluoride (Li/CFx) battery, the lithium-brominechloride (Li/BCx) battery, the lithium sulfuryl chloride (Li/SCl)battery and the lithium-silver vanadium oxide (Li/SVO) battery. Thesecondary battery is rechargeable and has low internal resistance thatis relatively stable over time. It will typically also have relativelylow energy density. Examples include the lithium-ion battery whichtypically utilizes lithium cobalt oxide and carbon. The primary andsecondary batteries are electrically connected in an indirect parallelarrangement by means of a voltage boost/charge control circuit designedto optimize the performance of the secondary battery for high energyimplantable medical use by limiting its maximum charge state anddischarge excursions.

In this indirect parallel connection arrangement of the primary andsecondary batteries, a voltage boost/charge control circuit isinterposed to increase the voltage supplied by the primary battery andto control the flow of energy supplied by the primary battery to thesecondary battery. This voltage boost/charge control circuit operatesindependently of primary battery run-down. For example, in a hybridbattery power source comprising an Li/CFx primary battery and alithium-ion secondary battery interconnected by a voltage boost/chargecontrol circuit, the principal energy supplied by the Li/CFx battery atabout 3.0 volts can be increased to charge the lithium-ion battery toits normal open circuit voltage of about 4.0 volts, if desired. Thischarging voltage can be sustained throughout the life of the device evenas the Li/CFx battery voltage falls during discharge to its typical endof life value of 2.4 volts. In this way, the primary battery is used asa long term energy supply to recharge the secondary battery for servicewhen and if its voltage drops below the desired level. The voltageboost/charge control circuit also serves to limit the current drawn fromthe primary battery during defibrillatory charging, ensuring thatvirtually all of the energy is drawn from the secondary battery. Thisprotects the primary battery from excessive high rate discharge thatcould lead to premature failure.

A third benefit provided by the voltage boost/charge control circuit isthe regulation of the charging current and voltage that is supplied tothe rechargeable secondary battery from the primary battery. In order tomaximize secondary battery performance and minimize the risk ofcatastrophic cell failure, the voltage and current must be closelyregulated throughout the life of both batteries, even as the primarybattery voltage decays through normal discharge. In particular, thelong-term performance of the secondary battery can be stronglyinfluenced by the maximum state of charge (SOC) maintained on thebattery and the discharge excursions that the battery experiences duringuse. For example, recharging and storing a lithium-ion secondary batteryat or near 100% SOC and allowing significant discharge before theinitiation of recharge can significantly increase the rate and extent towhich the battery energy capacity irreversibly fades during batteryservice life. Irreversible capacity fade in a lithium-ion batteryreduces its available energy output, which could decrease the number ofhigh energy pulses available from an implantable medical device.Capacity fade has also been correlated to long term internal resistanceincreases. Though less than the internal resistance variance found in atypical Li/SVO primary cell designed for implantable medical use, theinternal resistance increase of a lithium-ion secondary cell couldnonetheless impact the high energy pulse generating cycle of animplantable medical device. As such, the voltage boost/charge controlcircuit is adapted to limit the maximum SOC and allowable dischargeexcursions of the secondary battery to optimal levels.

The hybrid power source embodiments disclosed herein can be designed formaximum service life by selecting the primary and secondary batteries sothat the former provides most of the combined battery capacity whilestill allowing the latter to power a reasonable number of defibrillatorycharging cycles prior to recharging, say 50 cycles. By way of example,the secondary battery could be selected to provide 10% of the totalcapacity of the power source, with the primary battery providing theremaining 90%.

If an implantable defibrillator or ICD is provided with a Li/CFx primarybattery and a lithium-ion secondary battery arranged in the mannerdisclosed herein, the lithium-ion battery will be the predominant energysource for charging the defibrillator's energy storage capacitors duringthe defibrillation cycle. Since this battery would always be charged toan energy capacity sufficient to support a predetermined number ofdefibrillatory cycles, the charge state and voltage of the Li/CFxprimary battery would not be a factor in the ability of the device toquickly deliver defibrillation impulses. Thus, advantage can be taken ofthe superior energy density properties of the Li/CFx battery while alsoproviding the high rate discharge capabilities required fordefibrillators and ICDs. The manufacturer can rely on a lithium-ionbattery having no voltage delay properties of its own, and comparativelystable internal resistance and charge state, to supply thedefibrillation energy rapidly and consistently.

An additional benefit of this use of a lithium-ion secondary battery inconjunction with a voltage boost/charge control circuit is the highersource voltage available to the defibrillator inverter circuitry thatcharges the energy storage capacitors. The higher source voltagemitigates the effects of circuit current/resistance losses in theinverter that become more significant as the source voltage decreasesThe lithium-ion battery voltage is typically 3.8 to 4.0 volts as opposedto 2.0 to 3.0 volts for a Li/SVO cell of the type presently used inimplantable defibrillators and ICDs. Also, the lithium-ion cell providesa significant advantage over other lithium chemistries such as LiV₂O₅ at2.8 volts and LiTiS₂ at 2.2 volts, which are identified as candidatesecondary batteries in U.S. Pat. No. 5,674,248 of Kroll et al.,discussed by way of background above.

Illustrated Embodiments

Referring now to the Drawings in detail wherein like reference numeralshave been used throughout the various figures to designate likeelements, there is shown in FIG. 1 an illustration of two segments of ahybrid battery power source 2, namely, a primary battery “A” and asecondary battery “B”. By way of example only, the primary battery “A”will be assumed to incorporate either a Li/CFx or Li/SVO batterychemistry, and the secondary battery “B” will be assumed to incorporatea rechargeable lithium-ion chemistry. Such batteries are standardcommercial products that are available from Wilson GreatbatchTechnologies, Inc. of Clarence, N.Y. as catalog items. FIG. 1 alsodepicts that for each of battery “A” and battery “B”, one electricalconnection can be made via the vertical pins 4 and 6 respectivelyprotruding from the top of cases 8 and 10. The pins 4 and 6 representthe positive (cathode) terminals of the batteries. A second electricalconnection can be made to the cases 8 and 10 themselves, and casecontacts 12 and 14 are respectively shown. These case contacts representthe negative (anode) terminals of the batteries.

Turning now to FIG. 2, an exemplary hybrid battery power source 20 isshown. This power source includes the primary battery “A” and thesecondary battery “B” of FIG. 1. Battery “A” and battery “B” areconnected in parallel with a switching-type voltage boost/charge controlcircuit 22 interposed between the two batteries to charge the secondarybattery “B” to a higher potential than the primary battery “A” and toregulate the applied charging current and maximum cell charge/dischargevoltage excursions. By way of example, a 3.0-volt Li/CFx or 2.6-voltLi/SVO primary battery “A” will have its voltage boosted to about 3.8 to4.0 volts for recharging a lithium-ion secondary battery “B.” Within thecircuit 22 is a conventional oscillator-type pulse control circuit 24that provides variable width/duty cycle charging pulses to charge thesecondary battery “B” to a relatively constant potential even though thevoltage level of the primary battery “A” may vary through some range. Byway of example, if the primary battery “A” is an Li/SVO or Li/CFxbattery and the secondary battery “B” is a lithium-ion battery, thecharging voltage on the secondary battery “B” can be maintained at afairly constant level of 3.8 to 4.0 volts even though the output of theprimary battery “A” varies between about 2.0 to 3.0 volts during itsservice life. Thus, within the life of the primary battery “A,” everydefibrillator charging cycle will occur at nearly the same rate due tothe regulated charge state and, hence, constant voltage output of thesecondary battery “B.”

In FIG. 2, the primary battery “A” delivers energy to the input of thecircuit 22 while the output of the circuit 22 is connected to thesecondary battery “B.” The basic principle of operation for the circuit22 is as follows:

-   -   The pulse control circuit 24 derives prime power from the        primary battery “A” and includes an oscillator function. The        control circuit's output labeled “Switch” is toggled rapidly        (e.g. 100 kHz) to turn the field effect transistor Q1 on and        off.    -   Each time that transistor Q1 is turned on, current flow        increases in the inductor L1 and energy is imparted to the        magnetic field associated with the inductor. A rectifier diode        D1 is reverse biased and prevents current flow from the        secondary battery “B” backward in to Q1.    -   When transistor Q1 is turned off, the magnetic field collapses        in the inductor L1 and returns the energy to the circuit,        causing the voltage across the inductor to increase.    -   The collapsing magnetic field in the inductor develops a higher        positive voltage at the anode of rectifier diode D1. The diode        D1 becomes forward biased and current flows from the inductor L1        through the diode and into the secondary battery “B,”        transferring energy to that battery.    -   The circuit labeled “Feedback” samples the voltage at the        secondary battery “B” and controls the rate of switching of        transistor Q1 to regulate the flow of energy to battery “B” and,        hence, the charge imparted to that battery.    -   This feedback voltage is also applied to a conventional window        voltage comparator 25 which compares the battery “B” voltage        against a predetermined reference voltage “Vref” which, by way        of example, might be 3.9 volts. The comparator 25 is implemented        with hysteresis such that its output will change state only        after the “Feedback” voltage at the input has traversed a        predetermined voltage threshold window, e.g. 3.8 to 4.0 volts.        This function provides the control required via the circuit        labeled “Enable” to enable the pulse control circuit 24 only if        the battery “B” voltage falls below a minimum value and to        continue to charge battery “B” only until its terminal voltage        has risen to a maximum acceptable value. At that point the        “Enable” signal will be negated to prevent overcharging of        battery “B”.

As indicated, the primary benefit of the hybrid battery power source 20of FIG. 2 is that the secondary battery “B” can be charged andmaintained at a higher working voltage than the primary battery “A,” andvirtually all of the capacitor charging energy can be derived from thesecondary battery. By using voltage boosting and control circuitryhaving an inductive component connected between the primary andsecondary batteries, increased voltage is delivered to the secondarybattery “B” for recharging it to above the voltage of the primarybattery “A.” Moreover, the charging current is controlled by means of avariable pulse width and/or duty cycle control, allowing it to operateover a range of voltages on the primary battery “A,” such as about 2.0volts to 3.0 volts for an Li/SVO or Li/CFx battery, in all casescharging the secondary battery “B” to a higher voltage, such as 3.8 to4.0 volts for a lithium-ion battery. Thus, throughout the life cycle ofthe primary battery “A,” the secondary battery “B” will be deliveringthe same voltage to charge the capacitors in the defibrillator or ICD.This represents a significant advantage over prior art power sources,both in the short charge time and in the constancy of the charge goingto the secondary battery “B.” The full charge capacity of the primarybattery “A” is thus available. The pulse width and/or duty cycle of thepulse control circuit 24 is simply varied to maintain a constantcharging voltage on the secondary battery “B” regardless of where theprimary battery “A” is in its life cycle. Note that for a lithium-ionsecondary battery “B” charged to 4.0 volts and operating at a current of3.0 amperes with a conversion efficiency of 67 percent, a five secondcharge time is all that would be required to develop a 40 joule pulse.Again, this minimal charge time is a distinct improvement over previousdefibrillator batteries.

Another benefit of the hybrid battery power source 20 that has not beenpreviously disclosed in the prior art is the capability to regulate theflow of recharge energy from the primary battery “A” to the secondarybattery “B.” Regulation of the rechargeable battery charge state is anecessity for optimum energy storage because of the effects ofirreversible discharge capacity fade and increased cell internalresistance as a lithium-ion rechargeable cell is cycled. In particular,as reported by Spotnitz, R., “Simulation of Capacity Fade in Lithium-ionBatteries”, Journal of Power Sources, 113, 72–80, (2003, ), the maximumSOC applied to a lithium-ion cell as well as the depth of dischargeaffects capacity fade and internal resistance. In conventionalrechargeable battery applications where the batteries may be readilyreplaced, the practice is to maximize the charge and depth of dischargein order to obtain maximum output capacity from the battery. However, inhigh energy medical device applications, applicants have determined thatlimiting maximum SOC and depth of discharge are necessary in order toensure optimal pulse delivery characteristics. For a lithium-ion cell,charging to a maximum SOC of 4.0 V and attempting to limit the depth ofdischarge (ΔSOC) to 0.2 V (3.8 V minimum SOC) should provide adequateenergy output while minimizing capacity fade and internal resistanceincrease over time.

Referring now to FIG. 3A, a graph is shown of the discharge capacityretention versus the number of charge/discharge cycles for arepresentative lithium-ion battery. When first placed into service thebattery provides a maximum energy storage capacity that diminishes asthe battery is subjected to charging and discharging cycles. Reduceddischarge capacity in the secondary battery will manifest itself as areduction in the number of defibrillator charges that may be deliveredbefore the secondary battery requires charging. This degradation indischarge capacity is a strong function of the maximum SOC and depth ofdischarge that the battery is subjected to in each cycle. Thisdegradation can be minimized by configuring the voltage boost/chargingcontrol circuit 20 to limit the charge/discharge excursions during thedevice life.

Referring now to FIG. 3B, a graph is shown of the cell internalresistance versus the number of charge discharge cycles for arepresentative lithium-ion battery. When first placed into service thebattery exhibits minimum internal resistance that increases as thebattery is subjected to charging and discharging cycles. Increased cellinternal resistance in the secondary battery will manifest itself as anincrease in the time required to fully charge the defibrillator energystorage capacitors because of the diminished ability to deliver highdischarge currents. This increase in cell internal resistance is again astrong function of the maximum SOC and depth of discharge that thebattery is subjected to in each cycle. This degradation can therefore beminimized by configuring the voltage boost/charging control circuit 20to limit the charge/discharge excursions during the device life.

Turning now to FIG. 4, another exemplary hybrid battery power source 30is shown. This power source includes the primary battery “A” and thesecondary battery “B” of FIG. 1. Battery “A” and battery “B” areconnected in parallel with a voltage boost/charge control circuit 32containing a conventional oscillator-type pulse control circuit 34, butwith the voltage boost function being implemented with a flybackconverter. In particular, instead of utilizing a series inductor toboost the charging voltage to battery “B,” a transformer T1 with primaryand secondary connections is used. Although this represents a minorcomplication in circuitry, it permits complete isolation of the circuitsconnected to the primary and secondary sides of the transformer. In FIG.4, the primary battery “A” again provides the prime power for thecircuit, and its output is connected to the input of the circuit 32. Thepulse output of the circuit 32 charges the secondary battery “B.” Thebasic principles of operation for the circuit 32 are as follows:

-   -   The pulse control circuit 34 derives prime power from the        primary battery “A” and again includes an oscillator function.        The control circuit's output labeled “Switch” is toggled rapidly        (e.g. 100 kHz) to turn the field effect transistor Q1 on and        off.    -   Each time that transistor Q1 is turned on, the current in the        primary winding of transformer T1 increases and energy is stored        in the magnetic field associated with the transformer. The        connection of the secondary winding is reversed in the sense        that the induced voltage at the anode of rectifier diode D1 is        negative such that the rectifier is reverse biased and no        current flows in the secondary winding while the magnetic field        is increasing. The number of turns for the secondary winding may        be larger than that of the primary in order to provide a voltage        boost function.    -   When the control circuit 34 turns off transistor Q1 the magnetic        field begins to collapse, causing voltages of opposite polarity        to be induced across both the primary and secondary windings.        Because these voltages are opposite in polarity, the anode of        diode D1 is driven to a positive polarity, causing this        rectifier to conduct and current to flow, thereby delivering        energy to secondary battery “B.”    -   The circuits labeled “Feedback” sample the voltage of the        secondary battery “B” and are used by the pulse control circuit        34 to vary the rate of switching of transistor Q1. This provides        the means to regulate the flow of energy to the secondary        battery “B” and, hence, the charge imparted to that battery.    -   This feedback voltage is also applied to a differential        amplifier 35 whose output is 30 provided to a conventional        window voltage comparator 36. The differential amplifier 35        senses the terminal voltage of battery “B” while maintaining        electrical isolation between the negative circuits of batteries        “A” and “B”. The voltage output of differential amplifier 35 is        representative of the terminal voltage of battery “B” regardless        of any voltage difference between the battery “A” and battery        “B” negative circuits.    -   Window voltage comparator 36 compares the battery “B” voltage        against a predetermined reference voltage “Vref” which, by way        of example, might be 3.9 volts. The comparator 36 is implemented        with hysteresis such that its output will change state only        after the “Feedback” voltage at the input has traversed a        predetermined voltage threshold window, e.g. 3.8 to 4.0 volts.        This function provides the control required to enable the pulse        control circuit 34 only if the battery “B” voltage falls below a        minimum value and to continue to charge battery “B” only until        its terminal voltage has risen to a maximum acceptable value. At        that point the “Enable” signal will be negated to prevent        overcharging of battery “B”.

The primary benefit of the hybrid battery power source 30 of FIG. 4relative to the FIG. 3 design is that the primary and secondarybatteries “A” and “B” do not share a common return circuit.

Turning now to FIG. 5, a battery powered high energy medical device isembodied as a defibrillation system 40 that is shown schematically toinclude a hybrid battery power source 42 and other circuits and devicesfor defibrillating a heart 44 using high voltage pulses. The hybridbattery power source 42 includes the primary and secondary batteries “A”and “B” described above. Batteries “A” and “B” are preferably connectedin parallel via one of the voltage boost/charge control circuits 22 or32 as described above relative to FIGS. 2 and 4. The defibrillationsystem 40 includes physiologic electrodes 46 that are attached tocardiac tissue of the heart 44 for the purposes of monitoring cardiacactivity and delivering electrical energy in the event that cardiacactivity becomes abnormal. The electrodes are connected to thedefibrillation system 40 via conventional leads 48. A conventionalvoltage amplification system or flyback converter circuit 50 having anoscillator-type control circuit 52 therein is also provided as part ofthe defibrillator system 40. A description of defibrillation operationfor the depicted subsystems follows:

-   -   Circuitry (not shown) that is part of the defibrillation system        40 will monitor the activity of the cardiac muscle. Should the        monitored cardiac activity indicate the need for delivery of a        defibrillation charge, the signal labeled “Charge Enable” will        be asserted to activate the control circuit 52 that is part of        the flyback converter circuit 50. This will begin a capacitor        charge sequence for charging energy storage capacitors C2 and        C3.    -   The control circuit 52 derives prime power from the batteries        “A” and “B” and includes an oscillator function. Capacitor C1        provides high frequency decoupling of, and lowers the AC source        impedance for, the hybrid battery power source 42 to the flyback        converter 50. As stated, the hybrid battery power source 42 can        be constructed in accordance with any of the embodiments of        FIGS. 2 and 4, or modifications thereof. The control circuit        output labeled “Switch” is toggled rapidly (e.g. 100 kHz) to        turn transistor Q1 on and off.    -   Each time that the transistor is turned on, the current in the        primary winding of transformer T1 increases and energy is stored        in the magnetic field associated with the transformer. The        connection of the secondary windings is reversed in the sense        that the induced voltages at the anodes of rectifier diodes D1        and D2 is negative such that the diodes are reverse biased and        no current flows in the secondary windings while the magnetic        field is increasing.    -   When the control circuit 52 turns off transistor Q1 the magnetic        field begins to collapse, causing voltages of opposite polarity        to be induced across both the primary and secondary windings.        Because these voltages are opposite in polarity, the anodes of        diodes D1 and D2 are driven to a positive polarity, causing the        diodes to conduct and current to flow, delivering energy to        energy storage capacitors C2 and C3.    -   The number of turns in the secondary windings is much larger        than that of the primary, by a ratio of perhaps 100 to 1, in        order to achieve a large voltage boost function. Because of the        large turns ratio, the low primary winding voltage will be        increased to a high secondary winding voltage, the        multiplication factor being nearly equal to the turns ratio. In        this manner, a four volt prime power source may be increased to        hundreds of volts.    -   Two galvanically isolated secondary windings on transformer T1        have an identical number of turns and are each connected to        energy storage capacitors C2 and C3. This connection causes the        capacitors to be simultaneously charged in a parallel manner.    -   At the completion of the charge cycle, the defibrillation system        control circuitry negates the signal labeled “Charge Enable” and        asserts the signal labeled “Defibrillate Command.” This signal        activates the high voltage switch 54 depicted as two switch        circuits S1 a and S1 b. These switch circuits connect the energy        storage capacitors C2 and C3 in a series configuration to the        wires leading to the physiologic electrodes attached to the        cardiac tissue. The energy stored in the capacitors C2 and C3 is        rapidly delivered to the heart to provide the therapeutic        benefit.

The defibrillator system of FIG. 5 thus utilizes a flyback converter 50that has primary and secondary transformer windings in the circuitproviding isolation from the hybrid battery power source 42. This allowsa modification in which multiple transformers can be used, with theprimary sides thereof being connected in series or parallel and thesecondary sides thereof likewise being connected in series or parallel.This would provide voltage multiplication in case it is desired tomultiple capacitor banks.

Rationale for Configuration

The configuration of batteries and circuitry described above inconnection with the various drawing figures, provides an improvement inthe performance of implantable defibrillators and ICDs by reducingcharge times to manageable and predictable levels. These configurationsprovide the additional benefit of utilizing the stored energy of theprimary battery to maximum extent, thereby increasing the service lifefor the defibrillator system. As previously described herein, afundamental requirement for the power source in an implantabledefibrillator/ICD application is the ability to deliver a large amountof energy to the circuitry rapidly in order to charge the energy storagecapacitors in the shortest time possible. A second requirement of nearlyequal importance is the maximum utilization of stored energy within thepower source in order to provide maximum service life for the implanteddevice.

The rate at which power can be delivered from a battery or otherelectrical energy source to a load is inversely proportional to theinternal resistance or impedance of the energy source. This is due tothe fact that the load current flows through the internal resistance ofthe battery and the resulting power is dissipated as waste heat withinthe battery structure. In order to reduce the time required to chargethe energy storage capacitors, the charging circuit must draw higherload current from the battery. Both the Li/SVO and Li/CFX batterychemistries produce an internal resistance that renders them less ableto supply high peak currents as well as the lithium-ion battery. Thiscan be seen in Table 1 below.

TABLE 1 Lithium-CFx Lithium-SVO Lithium-Ion Battery Chemistry (primary)(primary) (rechargeable) Voltage Range (VDC)  2.8–2.0 3.0–2.0 4.0–3.5Internal Resistance >>1.0 0.5–1.0 0.1–0.5 (ohms) Energy Density (Wh/cc) 0.8–0.9 0.40 0.2

The internal resistance values given above are average values. It willbe seen that the internal resistance of the Li/CFx battery chemistry isthe highest, and this internal resistance renders such batteriesunsuitable for defibrillators and ICDs if used alone. As noted above,the internal resistance of the Li/SVO battery can increase tounacceptable levels during battery run-down. These batteries are thussusceptible to voltage delay effects, it may be concluded that they aresomewhat unreliable for implantable defibrillator and ICDs if usedalone.

By comparison, the internal resistance of the lithium-ion battery is lowand relatively stable. However, it will be seen that its energy densityis also substantially lower than that of Li/SVO and Li/CFx batteries. Inregard to battery energy density and device service life, it is knownthat for a given battery volume, the highest energy density battery willpossess the largest total energy and will, logically, provide thelongest device service life. Based upon energy density, the data inTable 1 indicates that the Li/CFx battery chemistry should provide thelongest device service life, with the Li/SVO battery chemistry being thesecond best. However, as indicated, the internal resistancecharacteristics of these batteries render them problematic if used alonein a defibrillator or ICD. The lowest energy density of the above-listedbattery chemistries is found in the lithium-ion battery. The energydensity level is so low that such batteries cannot realistically be usedin implantable defibrillators and ICDs, notwithstanding their favorableinternal resistance values.

The present invention solves these problems by utilizing two differentbattery chemistries wherein each battery is utilized in a manner whichis optimally matched to its capabilities. An Li/CFx battery, Li/BCx,Li/SCl or a Li/SVO battery can thus be chosen for the primary energysource (battery “A”) because they provide superior total energy for agiven battery volume. A lithium-ion battery can be chosen as the energysource for the energy storage capacitors (battery “B”) because itprovides an internal resistance that is low and relatively stable, andis therefore capable of delivering the highest instantaneous power,providing the shortest charging time for the energy storage capacitors.The embodiments that are taught herein thus provide a clear advantageover single battery systems because each battery is used in an optimumfashion, yielding an implantable defibrillator or ICD with minimumcapacitor charging time and maximum device service life.

The present invention is also superior with respect to preciseend-of-life determination of the power source. In particular, amonitoring feature could be provided that monitors the voltage level ofthe primary battery “A.” Over time, this voltage level may drop belowthe level necessary to trigger the voltage boost circuit. Monitoring thevoltage level of the primary battery “A” would provide a warning thatthe secondary battery “B” is no longer being charged. However, if thesecondary battery “B” is a lithium-ion cell, it will have a knowncapacity to deliver some number of defibrillatory pulses, say 50 pulses.It would thus be known that the defibrillator or ICD will work for thisremaining number of pulses and steps can be taken to promptly replacethe primary battery “A.”

This present invention is also superior to the concepts disclosed inprior art that embodies multi-battery systems with respect to themanagement of energy transfer from a primary energy stage to a secondaryenergy stage. Whereas the cited prior art teaches the use of simplecontinuous trickle charging circuits or the use of voltage doublercircuits in conjunction with a second stage rechargeable battery, weteach the use of a switch mode voltage boost/charge control circuit withfeedback that provides recharging under controlled conditions tooptimize the performance of the lithium-ion chemistry system utilizedfor the secondary battery “B.”

Accordingly, a hybrid battery power source for implantable medical usehas been disclosed and the objects of the invention have been achieved.It should, of course, be understood that the description and thedrawings herein are merely illustrative, and it will be apparent thatthe various modifications, combinations and changes can be made of thesestructures disclosed in accordance with the invention. It should beunderstood, therefore, that the invention is not to be in any waylimited except in accordance with the spirit of the appended claims andtheir equivalents.

1. A hybrid battery power source for implantable medical use, comprising: a primary battery; a secondary battery connected to receive power from said primary battery; said secondary battery being adapted to power to an implantable medical device designed for high energy electrical stimulation of body tissue for therapeutic purposes; and a charge control circuit powered by said primary battery and adapted to charge said secondary battery while limiting charge/discharge excursions thereof to a voltage range whose upper end is less than the maximum state of charge of said secondary battery and whose lower end is more than the minimum state of charge of said secondary battery, said upper end and said lower end of said voltage range being selected to minimize discharge capacity fade and internal resistance increase during service of said secondary battery.
 2. A hybrid battery power source in accordance with claim 1, further including a voltage boost circuit that facilitates charging of said secondary battery at a voltage that is higher than a voltage output of said primary battery.
 3. A hybrid battery power source in accordance with claim 2, wherein said voltage boost circuit comprises an inductive element.
 4. A hybrid battery power source in accordance with claim 2, wherein said voltage boost circuit comprises a flyback transformer.
 5. A hybrid battery power source in accordance with claim 1, wherein said charge control circuit includes a voltage comparator adapted to initiate charging when said secondary battery falls below a minimum reference voltage.
 6. A hybrid battery power source in accordance with claim 1, wherein said charge control circuit includes a window voltage comparator adapted to initiate charging when said secondary battery falls below a minimum reference voltage and to terminate charging when said secondary battery is charged to a maximum reference voltage that is larger than said minimum reference voltage.
 7. A hybrid battery power source in accordance with claim 1, wherein said primary battery is selected from the group consisting of lithium-carbon monofluoride batteries and lithium-silver vanadium oxide batteries, wherein said secondary battery is selected from the group consisting of lithium-ion batteries, and wherein said voltage range is 4.0–3.8 volts.
 8. A hybrid battery power source in accordance with claim 1, wherein said primary battery and said secondary battery are interconnected in parallel via said charge control circuit.
 9. A hybrid battery power source in accordance with claim 1 wherein said charge control circuit is a pulse circuit adapted for variable pulse width or duty cycle control, thereby allowing it to operate over a range of voltages output by said primary battery.
 10. An implantable medical device for high energy electrical stimulation of body tissue for therapeutic purposes, comprising: a pair of electrical contacts adapted to provide electrical stimulation to body tissue; energy storage means adapted to provide electrical energy to said electrical contacts; switching means adapted to periodically interconnect said energy storage means to said electrical contacts; and a hybrid battery power source adapted to provide power to said energy storage means and including: a primary battery; a secondary battery connected to receive power from said primary battery and to provide power to said energy storage means; and a charge control circuit powered by said primary battery and adapted to charge said secondary battery while limiting charge/discharge excursions thereof to a voltage range whose upper end is less than the maximum state of charge of said secondary battery and whose lower end is more than the minimum state of charge of said secondary battery, said upper end and said lower end of said voltage range being selected to minimize discharge capacity fade and internal resistance increase during service of said secondary battery.
 11. An implantable medical device in accordance with claim 10, further including a voltage boost circuit that facilitates charging of said secondary battery at a voltage that is higher than a voltage output of said primary battery.
 12. An implantable medical device in accordance with claim 10, wherein said voltage boost circuit comprises an inductive element.
 13. An implantable medical device in accordance with claim 10, wherein said voltage boost circuit comprises a flyback transformer.
 14. An implantable medical device in accordance with claim 9, wherein said charge control circuit is a pulse circuit adapted for variable pulse width or duty cycle control, thereby allowing it to operate over a range of voltages output by said primary battery.
 15. An implantable medical device in accordance with claim 9, wherein said charge control circuit includes a voltage comparator adapted to initiate charging when said secondary battery falls below a minimum reference voltage.
 16. An implantable medical device in accordance with claim 9, wherein said charge control circuit includes a window voltage comparator adapted to initiate charging when said secondary battery falls below a minimum reference voltage and to terminate charging when said secondary battery is charged to a maximum reference voltage that is larger than said minimum reference voltage.
 17. An implantable medical device in accordance with claim 9, wherein said primary battery is selected from the group consisting of lithium-carbon monofluoride batteries and lithium-silver vanadium oxide batteries, and wherein said secondary battery is selected from the group consisting of lithium-ion batteries, and wherein said voltage range is 4.0–3.8 volts.
 18. An implantable medical device in accordance with claim 9, wherein said primary battery and said secondary battery are interconnected in parallel via said charge control circuit.
 19. A method for powering an implantable medical device designed for high energy electrical stimulation of body tissue for therapeutic purposes, comprising: providing a primary power source; providing a secondary power source and connecting it to receive power from said primary power source; connecting said secondary power source to power said implantable medical device; and periodically charging said secondary battery by way of said primary battery while limiting charge/discharge excursions of said secondary battery to a voltage range whose upper end is less than a maximum state of charge of said secondary battery and whose lower end is more than a minimum state of charge of said secondary battery, said upper end and said lower end of said voltage range being selected to minimize discharge capacity fade and internal resistance increase during service of said secondary battery.
 20. A method in accordance with claim 18, wherein secondary battery is charged at a voltage that is higher than a voltage output of said primary battery.
 21. A method in accordance with claim 18, wherein said secondary battery is charged via pulse charging using variable pulse width or duty cycle control, thereby allowing said secondary battery to be charged over a range of voltages output by said primary battery.
 22. A method in accordance with claim 18, wherein charging of said secondary battery is initiated when said secondary battery falls below a minimum reference voltage.
 23. A method in accordance with claim 18, wherein said charging of said secondary battery is initiated when said secondary battery falls below a minimum reference voltage and terminated when said secondary battery is charged to a maximum reference voltage that is larger than said minimum reference voltage.
 24. A method in accordance with claim 18, wherein said primary battery is selected from the group consisting of lithium-carbon monofluoride batteries and lithium-silver vanadium oxide batteries, wherein said secondary battery is selected from the group consisting of lithium-ion batteries, and wherein said voltage range is 4.0–3.8 volts.
 25. A method in accordance with claim 18, wherein said primary battery and said secondary battery are interconnected in parallel via a voltage boost/charge control circuit that performs said periodic charging of said secondary battery.
 26. A method in accordance with claim 24, wherein said voltage boost/charge control circuit comprises a pulse control circuit and an inductive element.
 27. A method in accordance with claim 24, wherein said voltage boost/charge control circuit comprises a pulse control circuit and a flyback transformer. 